qemu: Resolve Coverity FORWARD_NULL

Coverity points out it was possible to have a zero return from
qemuBuildRNGBackendProps thus not filling in 'props' and then
causing a NULL dereference on the next call.

src/qemu/qemu_command.c

@@ -6464,7 +6464,9 @@ qemuBuildRNGBackendProps(virDomainRNGDefPtr rng,
         break;
 
     case VIR_DOMAIN_RNG_BACKEND_LAST:
-        break;
+        virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                       _("unknown rng-random backend"));
+        goto cleanup;
     }
 
     ret = 0;